Masters Degrees (Physics)
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Browsing Masters Degrees (Physics) by Advisor "Roos, W. D."
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Item Open Access The determination of ternary segregation parameters using a linear heating method(University of the Free State, 2000-11) Asante, Joseph Kwaku Ofori; Roos, W. D.; Du Plessis, J.In this study the segregation behaviour of the ternary system Cu(lll ), Sb, Sn is investigated experimentally, as well as with the modified Darken segregation model. The model, which describes the kinetics as well as the equilibrium of segregation, had been used successfully in various studies of binary systems. A computer program based on this model was developed for ternary systems. A Cu(lll) single crystal was doped with low concentrations of 0,180 at% Sb and 0.133 at% Sn using evaporation and diffusion process.' The experimental results were gathered with the Auger electron spectroscopy technique. This technique was combined with a linear temperature ramp that makes it possible to obtain the segregation parameters in a single run. The traditional method requires various runs at different temperatures. The overlapping of Sb and Sn Auger peaks in the energy regions of interest necessitated the development of a method to successfully extract the true contributions of the elements from the measured spectra. It is clearly shown that the combination of Auger peaks is not linear and that the true contributions of Sb and Sn can be calculated if the peaks overlap in two energy regions and the standard spectra are available. The segregation profiles resulted from the Auger data show clearly the sequential segregation of the two elements (Sn and Sb). From the equilibrium conditions, it is also concluded that an interaction energy between Sb and Sn is present. By simulating the experimental results, using the theoretical Darken model, values for the segregation parameters can be obtained. The initial values for the fits are found mathematically (highenergy regions) and manually (low energy regions). The calculated profiles fit the experimental results very well. The present study confirms that Sn segregate first to the surface with Do = 1.58x10-5 m²s-¹ and E = 170 kJ/mol. Sb with a lower dimsion coefficient (Do = 1.93x10-8 m²s-¹ and E = 150 kJ/mol) segregates at higher temperatures. A further increase in temperature results in the stronger segregate Sb, (with a higher segregation energy ∆G = -74.6 kJ/mol) to displace the Sn (∆G = -59.0 kJ/mol) from the surface. From the simulations, it is clear that the maximum surface coverage for Sn is determined mainly by the attractive interaction (ΩSnCu = -8.25 kJ/mol) between Sn and Cu. The desegregation rate of Sn in this system is determined by the segregation rate of Sb. The segregation profile of Sb is similar to that in a binary system (Cu,Sb) with the desegregation rate of Sb much slower than the segregation rate. The study also shows definite attractive interaction between Sb and Cu (ΩSbCu= -17.05 kJ/mol) This trend was not observed in the studies of binary systems. There is, however, repulsive interaction between the segregates (ΩSnSb = 3.62 kJ/mol). The repeatability of the segregation parameters at different heating rates shows that this experimental method can be used successfully.Item Open Access 'n Ondersoek na die segregasie van fosfor en ander onsuiwerhede in 3Cr12 vlekvrye staal(University of the Free State, 2003-11) Vermaak, Christiaan; Roos, W. D.; Terblans, J. J.English: One of the main reasons for temper embrittlement in steel is the segregation of impurities like P to the grain boundaries. Segregation can be defined as the diffusion of atoms from the bulk to the surface and grain boundaries in such a way that the total Gibbs free energy is minimized. This means that segregation can take place against the concentration gradient, from a low concentration in the bulk to a high surface concentration. The chemical potential gradient is the driving force behind segregation. The aim of this study is to investigate the segregation behaviour of P and other impurities like S and Sn in 3Cr12 steel. A background theory is founded by using: (i) The semi infinite solutions to the Fick equations (ii) t½ and modified t½ models (iii) the modified Darken model. One of the advantages of the Darken model is that it supported both segregation kinetics and equilibrium behaviour. The multi component model for ternary alloys could be expanded to quaternary alloy systems in this study. Segregation kinetics as well as the equilibrium was described by making use of constant and linear temperature heating. Auger electron spectroscopy was used to investigate the S, P, Cr, N, and Sn segregation behaviour in a Fe matrix. A personal computer was used to control the Auger spectrometer as well as the constant and linear heating runs. Three commercial 3Cr12 samples was investigated during the study. They were numbered according to their P contend as 26P for the sample with 0.026wt% P, 32P for 0.032wt% P and 62P for the sample containing 0.062wt% P. The constant temperature runs indicate that Sn competes with Cr, N and Pin sample 26P. A definite correlation is visible between Cr and N in sample 32P while Sn and S compete with P in sample 62P. The constant and linear heating Darken simulation model was used to give a qualitative description of the experimental segregation behaviour. The behaviour of two segregating species were simulated in a Fe matrix, from which the influence of the segregation parameters could be demonstrated, namely. If the surface concentration of species 1 is higher than that of species 2 during segregation kinetics, it can be said that the diffusion coefficient of species 1 is higher than that of species 2. If the surface concentration of species 1 is less than that of species 2, then the diffusion coefficient of species 1 is less than that of species 2. If the surface concentration of species 1 is less than that of species 2 at equilibrium, then the segregation energy of species 1 is less than that of species 2. If the equilibrium surface concentrations are equal, the segregation energies are equal. When the surface concentration of species 1 is higher than that of species 2, then the segregation energy of species 1 is higher than that of species 2. It is possible to sort the segregation parameters in order of magnitude from the results of the experimental work and the constant and linear heating simulations. The diffusion coefficients of the species could be arranged from high to low (DN > DP > DSn = DS). The segregation energies of samples 26P and 32P could be arranged in the same order, namely ?GS < ?GS n< ?GPItem Open Access Oxidation of a segregated MoN layer grown on Fe(100)-3.5wt%Mo-N(University of the Free State, 2001-06) Conradie, Rochelle; Roos, W. D.; Swart, H. C.English: The oxidation behaviour of the segregated MoN layer on the Fe(100)-3.5wt% Mo-N substrate was investigated in this study. Previous studies suggested the synergetic segregation of the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. It has also been shown that the segregated Mo and N form a MoN surface compound. As an alloy element in stainless steels, the Mo aids in the inhibition of the oxidation and thus prevents corrosion Auger electron spectroscopy (AES) was used to obtain the experimental results. For this study the oxidation of a Fe(100) specimen and a Fe(100)-3.5wt% Mo-N specimen were investigated to establish a point of reference to describe the oxidation behaviour of the segregated MoN layer. Linear temperature ramping was used to segregate the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. The specimens were exposed to an oxygen environment at various temperatures. The partial pressure of the oxygen was monitored with a mass spectrometer and was kept constant at 2 x 10-10 torr. The Auger peak-to-peak heights for the relevant elements in the specimens were measured as a function of the exposure time. Upon oxidation, the low energy Fe AES peak (47 eV) undergoes shape changes. The iron oxide has a dual peak with 42 eV and 52 eV kinetic energy respectively. The Fe(100) specimen surface reacted rapidly with the oxygen environment at room temperature to form an iron oxide, as depicted by the change in the low energy Fe AES peak. The exposures performed at 100°C and 200°C also resulted in oxide formation although the extent of the oxidation decreased with an increase in the temperature. Above 300°C indication of the Mo and N reacting with the oxygen environment. At 100°C and 200°C less oxide formation was detected and above 300°C there was only oxygen adsorption. The segregated MoN layer had a markedly different response to the oxygen exposure. The oxygen exposure performed at room temperature had a strikingly different course of the 0 Auger peak-to-peak height increase compared to that of the Fe(100) and Fe(100)- 3.5wt% Mo-N specimens exposure at the same temperature. The segregated MoN layer retards the surface reaction. A hypothesis formulated describes the MoN layer as a perforated layer that has some Fe exposed. The oxygen reacts rapidly with the exposed Fe. Longer exposures result in the dissociation of the MoN layer and the desorption of the Mo03 and NxOy compounds from the surface. Once the layer has dissociated completely the Fe will continue to react as for the other specimens. Oxidation occurs up to 300°C and at higher temperatures no oxide formation is detected. The changes in the low energy Fe AES peak are used to calculate the fraction oxide and metal contributing to the peak by using the Linear Least Squares method. The low energy Fe AES peak cannot be used for thickness calculations as it is subject to the backscattering term. The experimental data suggests that the backscattering term is a function of the exposure time. A first approximation is to assume a linear change with time. This approximation was applied successfully to the room temperature oxidation of the segregated MoN layer, but the same function could not be applied to the other two specimens, The thickness of the oxide was calculated using the change in the high energy Fe AES peak intensity. The O2 sticking coefficient for the exposure of the Fe(100) and the exposure of the segregated layer was also calculated and the differences in the values were attributed to the effect of the dissociation of the MoN layer on the adsorption of the O2 on the specimen surface. there was no oxide formation detected and therefore there is only oxygen adsorption at these temperatures. The Fe(100)-3.5wt% Mo-N specimen showed similar oxidation behaviour as was seen for the Fe(100) specimen. At room temperature the surface of the specimen reacted rapidly with the oxygen environment to form an iron oxide. There was noItem Open Access Segregation in a Cu bicrystal(University of the Free State, 2010-11) Jafta, Charl Jeremy; Roos, W. D.; Terblans, J. J.A literature study showed that the rate of segregation to a Cu(110) surface is higher than to a Cu(111) surface. The difference is mainly due to a change in the vacancy formation energy which determines the diffusion coefficient. The diffusion coefficient is a very important factor during kinetic segregation and determines the flux of atoms to the surface. The experimental verification of the calculations is very difficult due to the high number of parameters involved during measurements. In this study, the segregation parameters for Sb in a Cu bi-crystal, with (111) and (110) surface orientations, were determined. A unique experimental setup and measuring procedure was used to determine the concentration of the segregant as a function of temperature. This setup ensures exactly the same experimental conditions for both orientations allowing the researcher to directly compare the segregation parameters. The Auger Electron Spectroscopy (AES) spectrometer, used to measure the Sb enrichment on the Cu bi-crystal surfaces, was specially modified for these measurements. The deflection plates in the primary e- gun were physically aligned, horizontally and vertically, relative to the laboratory frame of reference. A computer program was developed to control the deflection of the e- beam during the measurements. Because it was decided on diffusional doping of the crystal an annealing system was designed and built. The system is consistently successful in annealing specimens at high temperatures for long periods of time in non corrosive atmospheres. Because of concerns that the grain boundary can influence the segregation, a secondary study was done on the migration of grain boundaries in polycrystalline Cu specimens. These studies indicate the inhibition of grain boundary mobility with small additions of Sb. The concentration build up on both surface orientations was monitored while the crystal was heated linearly with time, at different rates. The experimental data were fitted using the Modified Darken Model. The extracted Sb segregation parameters, in the Cu(110) surface are m2.s-1, kJ.mol-1, kJ.mol-1 and kJ.mol-1, and in the Cu(111) surface are m2.s-1, kJ.mol-1, kJ.mol-1 and kJ.mol-1. With the experimental conditions kept constant for both surface orientations, it is seen that there is a definite change in the pre-exponential factor and activation energy which compares well with values in literature. The different pre-exponential factors allows the opportunity to calculate, for the first time, the difference in the change in entropy ( ) for the two surface orientations as J.mol-1K-1. A unique custom build annealing system and experimental method used in this study proved to be highly successful and a change in the Sb segregation parameters, as a function of surface orientation, were experimentally verified.Item Open Access Simulating ion sputtered depth profiles in Auger electron spectroscopy(University of the Free State, 2004-05) Yohannes Tesfamicael, Biniam; Roos, W. D.; Terblans, J. J.; Wang, J. Y.Recent developments in advanced materials technology are mainly based on the progress in surface and interface science. These surface and interface properties of materials greatly affect and control the overall properties of the materials. The reliable performance of multilayered thin- film structures in many technological applications like microelectronics, for instance depends upon the mechanical and chemical stability of the interfaces. Hence, appropriate study and analysis of the interfaces is an important aspect that has to be carried out with great precision. Depth profiling is one of the most powerful mechanisms in the analysis of surface and interfaces of thin multilayered structures. This depth profiling is accomplished by surface analytical techniques like AES and XPS accompanied by ion sputtering. The principal aim of this depth profiling is to investigate the distribution of elemental concentration with depth. The ion etching of the sample during the depth profiling, however, imposes some effects on the shape of the profile. The major causes for the profile distortion comes from Atomic mixing, Interface roughness, Information depth of the secondary emission and preferential sputtering in multicomponent systems. A model (MRI) that is often used in literature to simulate depth profiles in Auger electron spectroscopy takes into account the effect of atomic mixing, interface roughness and information depth. One of the radiation-induced factors limiting depth resolution is preferential sputtering. In this study the model was modified to incorporate the effect of preferential sputtering on the distortion of the depth profile. Although preferential sputtering is an exponential function it was treated as independent of the other contributing functions and in such a way as to add to the total depth resolution in quadrature, according to an error propagation law. One application of the model is in the determination of interdiffusion parameters in annealed multilayered thin film structures. In the experimental part of this study a Cu/Ni multilayer structure was evaporated onto a silicon substrate. The samples were annealed for different times in the temperature range 250 to 350ºC. This was followed by Auger depth profiling using Ar + sputtering with 3 keV primary ions at an angle 60º to the surface normal. Deconvolution of the overlapping Cu and Ni Auger spectra were performed followed by the calibration of the depth and concentration scales. In the process of simulating the measured depth profile the modified model yielded the contributions of atomic mixing, information depth, interface roughness and the ratio of the sputtering yields of Cu and Ni. The value of the interface roughness, expected to be a function of annealing temperature and time, was used to calculate the interdiffusion coefficient. The diffusion parameters Do = 4x10 -14 m 2 /s and the activation energy Q=69kJ/mol agrees excellently with values available in literature where grain boundary diffusion is the dominant diffusion process. These results confirm the successful modification of the MRI model.Item Open Access TEM investigation of rapidly deformed Cu and Mo shaped charge liner material(University of the Free State, 2007-05) Cronje, Shaun; Kroon, R. E.; Roos, W. D.The strength and ductility of metals is a vast and important research area in which certain trends are well known, but where it is difficult to predict results with a high level of certainty, especially under extreme conditions e.g. high strain rates and very small grain sizes. Results may also be strongly influenced by impurities. All of the above factors play a vital role in the performance of shaped charge liners. Of particular interest is the material used in the manufacturing of liners. The microstructure and extended defects of copper and molybdenum shaped charge liners were investigated. Samples were extracted from the liners by electric discharge machining, to minimize any microstructural damage. Chemical testing revealed a higher than expected impurity concentration. Samples were annealed under two different annealing conditions, in order to obtain a variety of starting microstructures. Copper samples were annealed at 300°C for 30 minutes and 500°C for 30 minutes. Molybdenum samples were annealed at 1200°C for 30 minutes and 1200°C for 3 hours. These samples were then deformed at high strain rates using a split Hopkinson pressure bar. Two strain rates were used, the higher strain rate being approximately twice that of the lower strain rate. For both the copper and molybdenum the lower strain rate was on average 700 s-1, while the higher strain rate was on average 1550 s-1 and 1650 s-1 in the case of copper and molybdenum respectively. In the case of the molybdenum, the results showed a strong strain rate dependency of the yield strength which is typical of body centred cubic materials, whereas no such strain rate dependency could be detected in the copper results. Both materials show significant softening due to annealing, but relatively small changes between less and more intense annealing procedures. The unannealed samples showed significant variation in the stress-strain results, which is attributed to them originating from different parts of the liner. The uniformity of results after annealing indicates that the stress-strain properties of both materials after annealing are not strongly dependent on their prior straining history. The microstructure of these samples was examined using an optical microscope as well as a scanning electron microscope. The grain size was determined using the Heyn method. The as-received copper material had an elongated and heavily deformed microstructure. The lower annealing temperature produced a recrystallised grain structure, having an average grain size of 5 μm. The higher annealing temperature allowed grain growth with grains averaging 9 μm. The annealed copper samples contained annealing twins. In the case of molybdenum, the as-received material consisted of large (200 μm) grains. Annealing under both annealing conditions produced the same recrystallised, non-uniform grain structure with grains ranging from 47 μm to 92 μm. Transmission electron microscopy investigations of the samples revealed that deformation twinning occurred in the annealed and strained copper samples. This twinning occurred at a lower strain rate than expected. Dislocations in an annealed but unstrained copper sample occurred in entangled networks separated with areas containing no dislocations. These mixed dislocations were found to have Burgers vectors of the type b = a/2<110>. Pure edge dislocations with a [100] projected direction in the (110) plane with a Burgers vector of the type b = a/2[110 ] were also found. These dislocation arrays appear as ripple like structures. No evidence of twinning was found in the molybdenum samples. Some dislocations with Burger vectors of the type b = a/2<111> were found in the molybdenum samples. There are however exceptions, which is difficult to explain. This is an important observation, and further research would have to be performed.